Contact structures for three-dimensional memory device

ABSTRACT

Embodiments of contact structures of a three-dimensional memory device and fabrication method thereof are disclosed. The three-dimensional memory structure includes a film stack disposed on a substrate, wherein the film stack includes a plurality of conductive and dielectric layer pairs, each conductive and dielectric layer pair having a conductive layer and a first dielectric layer. The three-dimensional memory structure also includes a staircase structure formed in the film stack, wherein the staircase structure includes a plurality of steps, each staircase step having two or more conductive and dielectric layer pairs. The three-dimensional memory structure further includes a plurality of coaxial contact structures formed in a first insulating layer over the staircase structure, wherein each coaxial contact structure includes one or more conductive and insulating ring pairs and a conductive core, each conductive and insulating ring pair having a conductive ring and an insulating ring.

INCORPORATION BY REFERENCE

This application claims priority to PCT/CN2018/120715 filed on Dec. 12,2018, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to the field of semiconductortechnology, and more particularly, to a method for forming athree-dimensional (3D) memory.

BACKGROUND

Planar memory cells are scaled to smaller sizes by improving processtechnology, circuit designs, programming algorithms, and the fabricationprocess. However, as feature sizes of the memory cells approach a lowerlimit, planar process and fabrication techniques become challenging andcostly. As such, memory density for planar memory cells approaches anupper limit. A three-dimensional (3D) memory architecture can addressthe density limitation in planar memory cells.

BRIEF SUMMARY

Embodiments of contact structures for a three-dimensional memory deviceand methods for forming the same are described in the presentdisclosure.

In some embodiments, a three-dimensional memory structure includes afilm stack disposed on a substrate, wherein the film stack includes aplurality of conductive and dielectric layer pairs, each conductive anddielectric layer pair having a conductive layer and a first dielectriclayer. The three-dimensional memory structure also includes a staircasestructure formed in the film stack, wherein the staircase structureincludes a plurality of steps, each staircase step having two or moreconductive and dielectric layer pairs. The three-dimensional memorystructure further includes a plurality of coaxial contact structuresformed in a first insulating layer over the staircase structure, whereineach coaxial contact structure includes one or more conductive andinsulating ring pairs and a conductive core, wherein each conductive andinsulating ring pair includes a conductive ring and an insulating ring.

In some embodiments, each conductive ring contacts the conductive layerof a corresponding conductive and dielectric layer pair of the staircasestep.

In some embodiments, each coaxial contact structure comprises at leastan outer conductive ring and an inner conductive ring, and the outerconductive ring corresponds with an upper conductive and dielectriclayer pair of the staircase step, wherein the outer conductive ringincludes larger diameter, and the upper conductive and dielectric layerpair is farther away from the substrate.

In some embodiments, each coaxial contact structure comprises at leastan outer conductive ring and an inner conductive ring, and the innerconductive ring corresponds with a lower conductive and dielectric layerpair of the staircase step, wherein the inner conductive ring includessmaller diameter, and the lower conductive and dielectric layer pair iscloser to the substrate.

In some embodiments, the conductive core contacts the conductive layerclosest to the substrate in the staircase step of two or more conductiveand dielectric layer pairs.

In some embodiments, the insulating ring of the conductive andinsulating ring pair is disposed to surround a sidewall of theconductive ring and a sidewall of the conductive layer of the staircasestructure, wherein the insulating ring is configured to electricallyisolate the conductive ring from another conductive ring or theconductive core.

In some embodiments, the insulating ring is disposed on a sidewall ofthe first dielectric layer of the staircase step of two or moreconductive and dielectric layer pairs.

In some embodiments, the three-dimensional memory structure furtherincludes a barrier layer, disposed between the first insulating layerand the staircase structure, and the plurality of coaxial contactstructures extending through the barrier layer.

In some embodiments, the three-dimensional memory structure furtherincludes a gate dielectric layer on the conductive layer, and theconductive rings extending through the gate dielectric layer to contactthe conductive layers of the staircase structure.

Another aspect of the present disclosure provides a method for forming athree-dimensional (3D) memory device. A method for forming athree-dimensional (3D) memory structure includes disposing a dielectricfilm stack on a substrate, wherein the dielectric film stack includes aplurality of alternating dielectric layer pairs, each alternatingdielectric layer pair having a first dielectric layer and a seconddielectric layer different from the first dielectric layer. The methodalso includes forming a dielectric staircase in the dielectric filmstack, wherein the dielectric staircase includes a plurality of steps,each dielectric staircase step having two or more alternating dielectriclayer pairs. The method further includes disposing a first insulatinglayer on the dielectric staircase, forming a plurality of memory stringsin the dielectric film stack, and replacing the second dielectric layerswith conductive layers to form a staircase structure with a plurality ofsteps, wherein each staircase step includes two or more conductive anddielectric layer pairs, each conductive and dielectric layer pair havinga conductive layer and the first dielectric layer. The method alsoincludes forming a plurality of coaxial contact structures on thestaircase structure.

In some embodiments, forming the coaxial contact structure includesforming a conductive and insulating ring pair for each conductive anddielectric layer pair of the staircase step in the staircase structure.

In some embodiments, forming the conductive ring includes forming afirst contact hole to expose the conductive layer in one of the two ormore conductive and dielectric layer pairs of the staircase step in thestaircase structure, disposing a conductive film on a sidewall of thecontact hole and the exposed conductive layer, and removing theconducive film and a portion of the conductive layer from the bottom ofthe first contact hole to form a conductive ring, wherein a bottom ofthe conductive ring is formed to contact the conductive layer in one ofthe two or more conductive and dielectric layer pairs of the staircasestep in the staircase structure.

In some embodiments, forming the conductive ring further includesetching the dielectric layer of the next conductive and dielectric layerpair.

In some embodiments, forming the insulating ring includes disposing asecond insulating layer in a first contact hole, and removing the secondinsulating layer from the bottom of the first contact hole. Forming theinsulating ring also includes forming the insulating ring surrounding asidewall of the conductive ring and a sidewall of the conductive layerof one of the two or more conductive and dielectric layer pairs in thestaircase step in the staircase structure, and forming a second contacthole to expose the next conductive layer in the staircase step.

In some embodiments, forming the coaxial contact structure furtherincludes forming a contact hole to expose the conductive layer closestto the substrate in the staircase step of two or more conductive anddielectric layer pairs, disposing a conductive material to fill thecontact hole, and forming a conductive core to contact the conductivelayer closest to the substrate in staircase step of two or moreconductive and dielectric layer pairs.

In some embodiments, the method further includes performing aplanarization process to remove the conductive material outside thecontact hole and form a coplanar surface.

In some embodiments, the method further includes disposing a barrierlayer on the dielectric staircase prior to the first insulating layer.

In some embodiments, forming the plurality of dielectric staircase stepsincludes disposing a patterning mask on the dielectric film stack, andetching exposed portions of the dielectric film stack in a directionperpendicular to a main surface of the substrate until portions of thetwo or more dielectric layer pairs are removed. Forming the plurality ofdielectric staircase steps also includes trimming the patterning masklaterally, in a direction parallel to the main surface of the substrate,repeating the etching and the trimming processes until a dielectricstaircase step closest to the main surface of the substrate is formed,and removing the patterning mask.

In some embodiments, replacing the second dielectric layers with theconductive layers to form the staircase structure, includes forming oneor more slit structure openings, extending horizontally along thedielectric staircase, wherein the slit structure openings penetratevertically through the dielectric film stack. Replacing the seconddielectric layers also includes removing the second dielectric layers ofthe dielectric staircase to form a plurality of horizontal tunnels, anddisposing the conductive layers inside the plurality of horizontaltunnels.

In some embodiments, the method further includes disposing a gatedielectric layer on sidewalls of the horizontal tunnels prior todisposing the conductive layer, wherein the gate dielectric layerincludes high-k dielectric material, silicon oxide, silicon nitride orsilicon oxynitride.

Other aspects of the present disclosure can be understood by thoseskilled in the art in light of the description, the claims, and thedrawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate embodiments of the present disclosureand, together with the description, further serve to explain theprinciples of the present disclosure and to enable a person skilled inthe pertinent art to make and use the present disclosure.

FIG. 1 illustrates a schematic top-down view of an exemplarythree-dimensional (3D) memory die, according to some embodiments of thepresent disclosure.

FIG. 2A-2B illustrate schematic top-down views of some regions of 3Dmemory die, according to some embodiments of the present disclosure.

FIG. 3 illustrates a perspective view of a portion of an exemplary 3Dmemory array structure, in accordance with some embodiments of thepresent disclosure.

FIG. 4-15 illustrate schematic cross-sectional views of an exemplary 3Dmemory structure at certain fabricating stages, according to someembodiments of the present disclosure.

FIG. 16A illustrates a schematic cross-sectional view of an exemplary 3Dmemory structure at a certain fabricating stage, according to someembodiments of the present disclosure.

FIG. 16B illustrates a perspective view of a portion of an exemplary 3Dmemory structure at a certain fabricating stage, according to someembodiments of the present disclosure.

FIG. 17A-17D illustrate schematic cross-sectional views of an exemplary3D memory structure at certain fabricating stages, according to someembodiments of the present disclosure.

FIG. 18 illustrates a flow diagram of an exemplary method for forming a3D memory structure, according to some embodiments of the presentdisclosure.

The features and advantages of the present invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The drawing in which an elementfirst appears is indicated by the leftmost digit(s) in the correspondingreference number.

Embodiments of the present disclosure will be described with referenceto the accompanying drawings.

DETAILED DESCRIPTION

Although specific configurations and arrangements are discussed, itshould be understood that this is done for illustrative purposes only. Aperson skilled in the pertinent art will recognize that otherconfigurations and arrangements can be used without departing from thespirit and scope of the present disclosure. It will be apparent to aperson skilled in the pertinent art that the present disclosure can alsobe employed in a variety of other applications.

It is noted that references in the specification to “one embodiment,”“an embodiment,” “an example embodiment,” “some embodiments,” etc.,indicate that the embodiment described can include a particular feature,structure, or characteristic, but every embodiment can not necessarilyinclude the particular feature, structure, or characteristic. Moreover,such phrases do not necessarily refer to the same embodiment. Further,when a particular feature, structure or characteristic is described inconnection with an embodiment, it would be within the knowledge of aperson skilled in the pertinent art to affect such feature, structure orcharacteristic in connection with other embodiments whether or notexplicitly described.

In general, terminology can be understood at least in part from usage incontext. For example, the term “one or more” as used herein, dependingat least in part upon context, can be used to describe any feature,structure, or characteristic in a singular sense or can be used todescribe combinations of features, structures or characteristics in aplural sense. Similarly, terms, such as “a,” “an,” or “the,” again, canbe understood to convey a singular usage or to convey a plural usage,depending at least in part upon context. In addition, the term “basedon” can be understood as not necessarily intended to convey an exclusiveset of factors and may, instead, allow for existence of additionalfactors not necessarily expressly described, again, depending at leastin part on context.

It should be readily understood that the meaning of “on,” “above,” and“over” in the present disclosure should be interpreted in the broadestmanner such that “on” not only means “directly on” something, but alsoincludes the meaning of “on” something with an intermediate feature or alayer therebetween. Moreover, “above” or “over” not only means “above”or “over” something, but can also include the meaning it is “above” or“over” something with no intermediate feature or layer therebetween(i.e., directly on something).

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper,” and the like, can be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or process step in addition to the orientation depicted inthe figures. The apparatus can be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein can likewise be interpreted accordingly.

As used herein, the term “substrate” refers to a material onto whichsubsequent material layers are added. The substrate includes a topsurface and a bottom surface. The top surface of the substrate istypically where a semiconductor device is formed, and therefore thesemiconductor device is formed at a top side of the substrate unlessstated otherwise. The bottom surface is opposite to the top surface andtherefore a bottom side of the substrate is opposite to the top side ofthe substrate. The substrate itself can be patterned. Materials added ontop of the substrate can be patterned or can remain unpatterned.Furthermore, the substrate can include a wide array of semiconductormaterials, such as silicon, germanium, gallium arsenide, indiumphosphide, etc. Alternatively, the substrate can be made from anelectrically non-conductive material, such as a glass, a plastic, or asapphire wafer.

As used herein, the term “layer” refers to a material portion includinga region with a thickness. A layer has a top side and a bottom sidewhere the bottom side of the layer is relatively close to the substrateand the top side is relatively away from the substrate. A layer canextend over the entirety of an underlying or overlying structure, or canhave an extent less than the extent of an underlying or overlyingstructure. Further, a layer can be a region of a homogeneous orinhomogeneous continuous structure that has a thickness less than thethickness of the continuous structure. For example, a layer can belocated between any set of horizontal planes between, or at, a topsurface and a bottom surface of the continuous structure. A layer canextend horizontally, vertically, and/or along a tapered surface. Asubstrate can be a layer, can include one or more layers therein, and/orcan have one or more layer thereupon, thereabove, and/or therebelow. Alayer can include multiple layers. For example, an interconnect layercan include one or more conductive and contact layers (in whichcontacts, interconnect lines, and/or vertical interconnect accesses(VIAs) are formed) and one or more dielectric layers.

As used herein, the term “nominal/nominally” refers to a desired, ortarget, value of a characteristic or parameter for a component or aprocess step, set during the design phase of a product or a process,together with a range of values above and/or below the desired value.The range of values can be due to slight variations in manufacturingprocesses or tolerances. As used herein, the term “about” indicates thevalue of a given quantity that can vary based on a particular technologynode associated with the subject semiconductor device. Based on theparticular technology node, the term “about” can indicate a value of agiven quantity that varies within, for example, 10-30% of the value(e.g., ±10%, ±20%, or ±30% of the value).

In the present disclosure, the term“horizontal/horizontally/lateral/laterally” means nominally parallel toa lateral surface of a substrate. In the present disclosure, the term“each” may not only necessarily mean “each of all,” but can also mean“each of a subset.”

As used herein, the term “3D memory” refers to a three-dimensional (3D)semiconductor device with vertically oriented strings of memory celltransistors (referred to herein as “memory strings,” such as NANDstrings) on a laterally-oriented substrate so that the memory stringsextend in the vertical direction with respect to the substrate. As usedherein, the term “vertical/vertically” means nominally perpendicular tothe lateral surface of a substrate.

In the present disclosure, for ease of description, “tier” is used torefer to elements of substantially the same height along the verticaldirection. For example, a word line and the underlying gate dielectriclayer can be referred to as “a tier,” a word line and the underlyinginsulating layer can together be referred to as “a tier,” word lines ofsubstantially the same height can be referred to as “a tier of wordlines” or similar, and so on.

In some embodiments, a memory string of a 3D memory device includes asemiconductor pillar (e.g., silicon channel) that extends verticallythrough a plurality of conductive and dielectric layer pairs. Theplurality of conductive and dielectric layer pairs are also referred toherein as an “alternating conductive and dielectric stack.” Anintersection of the conductive layer and the semiconductor pillar canform a memory cell. The conductive layer of the alternating conductiveand dielectric stack can be connected to a word line at theback-end-of-line, wherein the word line can electrically connect to oneor more control gates. For illustrative purposes, word lines and controlgates are used interchangeably to describe the present disclosure. Thetop of the semiconductor pillar (e.g., transistor drain region) can beconnected to a bit line (electrically connecting one or moresemiconductor pillars). Word lines and bit lines are typically laidperpendicular to each other (e.g., in rows and columns, respectively),forming an “array” of the memory, also called a memory “block” or an“array block”.

A memory “die” may have one or more memory “planes”, and each memoryplane may have a plurality of memory blocks. An array block can also bedivided into a plurality of memory “pages”, wherein each memory page mayhave a plurality of memory strings. In a flash NAND memory device, eraseoperation can be performed for every memory block and read/writeoperation can be performed for every memory page. The array blocks arethe core area in a memory device, performing storage functions. Toachieve higher storage density, the number of vertical 3D memory stacksis increased greatly, adding complexity and cost in manufacturing.

A memory die has another region, called the periphery, which providessupporting functions to the core. The periphery region includes manydigital, analog, and/or mixed-signal circuits, for example, row andcolumn decoders, drivers, page buffers, sense amplifiers, timing andcontrols, and the like circuitry. Peripheral circuits use active and/orpassive semiconductor devices, such as transistors, diodes, capacitors,resistors, etc., as would be apparent to a person of ordinary skill inthe art.

Other parts of the memory devices are not discussed for ease ofdescription. In the present disclosure, a “memory device” is a generalterm and can be a memory chip (package), a memory die or any portion ofa memory die.

Although using three-dimensional NAND devices as examples, in variousapplications and designs, the disclosed structures can also be appliedin similar or different semiconductor devices to, e.g., to improve metalconnections or wiring. The specific application of the disclosedstructures should not be limited by the embodiments of the presentdisclosure.

FIG. 1 illustrates a top-down view of an exemplary three-dimensional(3D) memory device 100, according to some embodiments of the presentdisclosure. The 3D memory device 100 can be a memory die and can includeone or more memory planes 101, each of which can include a plurality ofmemory blocks 103. Identical and concurrent operations can take place ateach memory plane 101. The memory block 103, which can be megabytes (MB)in size, is the smallest size to carry out erase operations. Shown inFIG. 1, the exemplary 3D memory device 100 includes four memory planes101 and each memory plane 101 includes six memory blocks 103. Eachmemory block 103 can include a plurality of memory cells, wherein eachmemory cell can be addressed through interconnections such as bit linesand word lines. The bit lines and word lines can be laid outperpendicularly, forming an array of metal lines. The direction of bitlines and word lines are labeled as “BL” and “WL” in FIG. 1. In thisdisclosure, memory blocks 103 is also referred to as “memory arrays”.

The 3D memory device 100 also includes a periphery regions 105, an areasurrounding memory planes 101. The periphery region 105 containsperipheral circuits to support functions of the memory array, forexample, page buffers, row and column decoders and sense amplifiers.

It is noted that, the arrangement of the memory planes 101 in the 3Dmemory device 100 and the arrangement of the memory blocks 103 in eachmemory plane 101 illustrated in FIG. 1 are only used as an example,which does not limit the scope of the present disclosure.

In some embodiments, the memory arrays and the peripheral circuits ofthe 3D memory device 100 can be formed on different substrates and canbe joined together to form the 3D memory device 100 through waferbonding. In this example, through array contact structures can providevertical interconnects between the memory arrays and peripheralcircuits, thereby reducing metal levels and shrinking die size. Detailedstructure and method of 3D memory with hybrid bonding is described inco-pending U.S. patent application, titled “Hybrid Bonding ContactStructure of Three-Dimensional Memory Device,” (application Ser. No.16/046,852 and filed on Jul. 26, 2018), which is incorporated herein byreference in its entirety.

Referring to FIG. 2A, an enlarged top-down view of a region 108 in FIG.1 is illustrated, according to some embodiments of the presentdisclosure. The region 108 of the 3D memory device 100 can include astaircase region 210 and a channel structure region 211. The channelstructure region 211 can include an array of memory strings 212, eachincluding a plurality of stacked memory cells. The staircase region 210can include a staircase structure and an array of contact structures 214formed on the staircase structure. In some embodiments, a plurality ofslit structures 216, extending in WL direction across the channelstructure region 211 and the staircase region 210, can divide a memoryblock into multiple memory fingers 218. At least some slit structures216 can function as the common source contact for an array of memorystrings 212 in channel structure regions 211. A top select gate cut 220can be disposed in the middle of each memory finger 218 to divide a topselect gate (TSG) of the memory finger 218 into two portions, andthereby can divide a memory finger into two programmable (read/write)pages. While erase operation of a 3D NAND memory can be carried out atmemory block level, read and write operations can be carried out atmemory page level. A page can be kilobytes (KB) in size. In someembodiments, region 108 also includes dummy memory strings 222 forprocess variation control during fabrication and/or for additionalmechanical support.

Referring to FIG. 2B, an enlarged top-down view of a region 109 in FIG.1 is illustrated, according to some embodiments of the presentdisclosure. The region 109 of the 3D memory device 100 can include thechannel structure region 211, a through array contact region 107, and atop select gate (TSG) staircase region 224.

The channel structure region 211 in the region 109 can be similar to thechannel structure region 211 in region 108. The TSG staircase region 224can include an array of TSG contacts 226 formed on the staircasestructure. The TSG staircase region 224 can be disposed on the sides ofthe channel structure region 211 and adjacent to through array contactregion 107 in the top-down view. Multiple through array contacts 228 canbe formed in the through array contact region 107.

FIG. 3 illustrates a perspective view of a portion of an exemplarythree-dimensional (3D) memory array structure 300, according to someembodiments of the present disclosure. The memory array structure 300includes a substrate 330, an insulating film 331 over the substrate 330,a tier of lower select gates (LSGs) 332 over the insulating film 331,and a plurality of tiers of control gates 333, also referred to as “wordlines (WLs)”, stacking on top of the LSGs 332 to form a film stack 335of alternating conductive and dielectric layers. The dielectric layersadjacent to the tiers of control gates are not shown in FIG. 3 forclarity.

The control gates of each tier are separated by slit structures 216-1and 216-2 through the film stack 335. The memory array structure 300also includes a tier of top select gates (TSGs) 334 over the stack ofcontrol gates 333. The stack of TSG 334, control gates 333 and LSG 332is also referred to as “gate electrodes.” The memory array structure 300further includes memory strings 212 and doped source line regions 344 inportions of substrate 330 between adjacent LSGs 332. Each memory strings212 includes a channel hole 336 extending through the insulating film331 and the film stack 335 of alternating conductive and dielectriclayers. Memory strings 212 also includes a memory film 337 on a sidewallof the channel hole 336, a channel layer 338 over the memory film 337,and a core filling film 339 surrounded by the channel layer 338. Amemory cell 340 can be formed at the intersection of the control gate333 and the memory string 212. The memory array structure 300 furtherincludes a plurality of bit lines (BLs) 341 connected to the memorystrings 212 over the TSGs 334. The memory array structure 300 alsoincludes a plurality of metal interconnect lines 343 connected to thegate electrodes through a plurality of contact structures 214. The edgeof the film stack 335 is configured in a shape of staircase to allow anelectrical connection to each tier of the gate electrodes. The channelstructure region 211 and the staircase region 210 correspond to thechannel structure region 211 and the staircase region 210 in thetop-down view of FIG. 2A, wherein one of the staircase region 210 inFIG. 3 can be used as TSG staircase region 230 for TSG connection.

In FIG. 3, for illustrative purposes, three tiers of control gates333-1, 333-2, and 333-3 are shown together with one tier of TSG 334 andone tier of LSG 332. In this example, each memory string 212 can includethree memory cells 340-1, 340-2 and 340-3, corresponding to the controlgates 333-1, 333-2 and 333-3, respectively. In some embodiments, thenumber of control gates and the number of memory cells can be more thanthree to increase storage capacity. The memory array structure 300 canalso include other structures, for example, through array contact, TSGcut, common source contact and dummy channel structure. These structuresare not shown in FIG. 3 for simplicity.

With the demand for higher storage capacity in a NAND flash memory, thenumber of vertical tiers of 3D memory cells 340 or word lines 333increases accordingly, leading to more process complexity and highermanufacturing cost. When increasing the tiers of memory cells 340 orword lines 333 of the memory array structure 300, it becomes morechallenging to etch deeper channel holes 336 for the memory strings 212and also more challenging to form contact structures 214 on thestaircase structures. For example, to form the contact structures 214 ona large number of vertically stacked word lines (gate electrodes), ahigh aspect ratio etching is needed to form contact holes, followed by ahigh aspect ratio deposition of conductive materials inside the contactholes. To reduce cost per bit for a 3D memory, dimensions of the memorystructures are reduced to allow fabrication of more memory blocks on awafer. However the increased word line stack also leads to widerstaircase structures in a horizontal direction parallel to the substratesurface, resulting in a wider staircase region 210 and less storagedensity.

To alleviate etching and deposition difficulties with more and morevertically stacked word lines, portions of a 3D memory device can beformed on two or more wafers and then joined together through waferbonding or flip-chip bonding. Alternatively, a 3D memory device can beformed by sequentially stacking multi-sessions, wherein each sessioncontains a stack of word lines with less number of tiers. However largerlateral dimensions of staircase structures due to vertically stackedword lines still limits the storage density.

Various embodiments in the present disclosure describe a structure andmethod of a 3D memory with coaxial contact structures, each providingelectrical contacts to two or more conductive layers of the staircasestructure. By sharing contact structures between multiple conductivelayers, the dimension of the staircase region 210 (in FIG. 2) can bereduced. Memory density and cost per bit of the 3D NAND memory can beimproved accordingly.

FIG. 4 illustrates a cross-sectional view of an exemplary structure 400of a three-dimensional memory device, according to some embodiments,wherein the structure 400 includes a substrate 330 and a dielectric filmstack 445. The cross-sectional views of FIG. 4-17D are along WLdirection in FIG. 2A.

Substrate 330 can provide a platform for forming subsequent structures.In some embodiments, the substrate 330 includes any suitable materialfor forming the three-dimensional memory device. For example, thesubstrate 330 can include any other suitable material, for example,silicon, silicon germanium, silicon carbide, silicon on insulator (SOI),germanium on insulator (GOI), glass, gallium nitride, gallium arsenide,III-V compound, and/or any combinations thereof.

A front surface 330 f of the substrate 330 is also referred to as a“main surface” of the substrate herein. Layers of materials can bedisposed on the front surface 330 f of the substrate. A “topmost” or“upper” layer is a layer farthest or farther away from the front surface330 f of the substrate. A “bottommost” or “lower” layer is a layerclosest or closer to the front surface 330 f of the substrate.

In some embodiments, peripheral devices can be formed in the peripheryregion 105 on the front surface 330 f of the substrate 330. In someembodiments, active device areas can be formed in the memory blocks 103on the front surface 330 f of the substrate 330. In some embodiments,the substrate 330 can further include an insulating film 331 on thefront surface 330 f. The insulating film 331 can be made of the same ordifferent material from the dielectric film stack.

The peripheral devices can include any suitable semiconductor devices,for example, metal oxide semiconductor field effect transistors(MOSFETs), diodes, resistors, capacitor, etc. The peripheral devices canbe used in the design of digital, analog and/or mixed signal circuitssupporting the storage function of the memory core, for example, row andcolumn decoders, drivers, page buffers, sense amplifiers, timing andcontrols.

The active device areas in the memory blocks are surrounded by isolationstructures, such as shallow trench isolation. Doped regions, such asp-type doped and/or n-type doped wells, can be formed in the activedevice area according to the functionality of the array devices in thememory blocks.

The dielectric film stack 445 extends in a lateral direction that isparallel to the front surface 330 f of the substrate 330. The dielectricfilm stack 445 includes a dielectric layer 450 (also referred to as“first dielectric layer”) and a sacrificial layer 452 (also referred toas “second dielectric layer”) alternatingly stacked on each other,wherein the dielectric layer 450 is configured to be the bottommost andthe topmost layers of the dielectric film stack 445. In thisconfiguration, each sacrificial layer 452 is sandwiched between twodielectric layers 450, and each dielectric layer 450 is sandwichedbetween two sacrificial layers 452 (except the bottommost and thetopmost layer).

The dielectric layer 450 and the underlying sacrificial layer 452 arealso referred to as an alternating dielectric layer pair 454. Theformation of the dielectric film stack 445 can include disposing thedielectric layers 450 to each have the same thickness or to havedifferent thicknesses. Example thicknesses of the dielectric layers 450can range from 10 nm to 500 nm. Similarly, the sacrificial layer 452 caneach have the same thickness or have different thicknesses. Examplethicknesses of the sacrificial layer 452 can range from 10 nm to 500 nm.

Although only 21 total layers are illustrated in the dielectric filmstack 445 in FIG. 4, it should be understood that this is forillustrative purposes only and that any number of layers may be includedin the dielectric film stack 445.

In some embodiments, the dielectric film stack 445 can include layers inaddition to the dielectric layer 450 and the sacrificial layer 452, andcan be made of different materials and with different thicknesses.

In some embodiments, the dielectric layer 450 includes any suitableinsulating materials, for example, silicon oxide, silicon oxynitride,silicon nitride, TEOS or silicon oxide with F—, C—, N—, and/or H—incorporation. The dielectric layer 450 can also include high-kdielectric materials, for example, hafnium oxide, zirconium oxide,aluminum oxide, tantalum oxide, or lanthanum oxide films.

The formation of the dielectric layer 450 on the substrate 330 caninclude any suitable deposition methods such as, chemical vapordeposition (CVD), physical vapor deposition (PVD), plasma-enhanced CVD(PECVD), rapid thermal chemical vapor deposition (RTCVD), low pressurechemical vapor deposition (LPCVD), sputtering, metal-organic chemicalvapor deposition (MOCVD), atomic layer deposition (ALD),high-density-plasma CVD (HDP-CVD), thermal oxidation, nitridation, anyother suitable deposition method, and/or combinations thereof.

In some embodiments, the sacrificial layer 452 includes any suitablematerial that is different from the dielectric layer 450 and can beremoved selectively. For example, the sacrificial layer 452 can includesilicon oxide, silicon oxynitride, silicon nitride, TEOS,poly-crystalline silicon, poly-crystalline germanium, poly-crystallinegermanium-silicon, and any combinations thereof. In some embodiments,the sacrificial layer 452 also includes amorphous semiconductormaterials, such as amorphous silicon or amorphous germanium. Thesacrificial layer 452 can be disposed using a similar technique as thedielectric layer 450, such as CVD, PVD, ALD, thermal oxidation ornitridation, or any combination thereof.

In some embodiments, the dielectric layer 450 can be silicon oxide andthe sacrificial layer 452 can be silicon nitride.

FIG. 5 illustrates a cross-sectional view of an exemplary structure 500of a three dimensional memory device, according to some embodiments,wherein the structure 500 includes a dielectric staircase 560 formed inthe dielectric film stack 445. In the dielectric staircase 560, adielectric staircase step 562, or a “staircase layer”, refers to a layerstack with the same lateral dimension in a surface parallel to thesubstrate surface 330 f Each dielectric staircase step 562 terminates ata shorter length than the staircase step underneath, with a lateraldimension “a” shown in FIG. 5.

In some embodiments, each dielectric staircase step 562 includes two ormore alternating dielectric layer pairs 454. Each dielectric staircasestep 562 can have a same number of alternating dielectric layer pairs ora different number of alternating dielectric layer pairs. As an example,FIG. 5 depicts the dielectric staircase 560 with two alternatingdielectric layer pairs 454.

The plural steps of the dielectric staircase 560 can be formed byapplying a repetitive etch-trim process on the dielectric film stack 445using a patterning mask (not shown). In some embodiments, the patterningmask can include a photoresist or carbon-based polymer material. Thepatterning mask can be removed after forming the dielectric staircase560.

The etch-trim process includes an etching process and a trimmingprocess. During the etching process, a portion of each dielectricstaircase step 562 with exposed surface can be removed. The remainingportion of each dielectric staircase step 562, either covered by upperlevels of staircase steps or covered by the patterning mask, is notetched. The etch depth is a thickness of the dielectric staircase step562. In some embodiments, the thickness of the dielectric staircase step562 is the total thickness of two or more alternating dielectric layerpairs 454. In the example shown in FIG. 5, the thickness of a dielectricstaircase step 562 is the thickness of two alternating dielectric layerpairs 454. The etching process for the dielectric layer 450 can have ahigh selectivity over the sacrificial layer 452, and/or vice versa.Accordingly, an underlying alternating dielectric layer pair 454 canfunction as an etch-stop layer. By switching etching process for eachlayer, the dielectric staircase step 562 with multiple alternatingdielectric layer pairs 454 can be etched during one etching cycle. Andas a result, one staircase step is formed during each etch-trim cycle.

In some embodiments, the dielectric staircase step 562 can be etchedusing an anisotropic etching such as a reactive ion etch (RIE) or otherdry etch processes. In some embodiments, the dielectric layer 450 issilicon oxide. In this example, the etching of silicon oxide can includeRIE using fluorine based gases, for example, carbon-fluorine (CF₄),hexafluoroethane (C₂F₆), CHF₃, or C₃F₆ and/or any other suitable gases.In some embodiments, the silicon oxide layer can be removed by wetchemistry, such as hydrofluoric acid or a mixture of hydrofluoric acidand ethylene glycol. In some embodiments, a timed etching approach canbe used. In some embodiments, the sacrificial layer 452 is siliconnitride. In this example, the etching of silicon nitride can include RIEusing O₂, N₂, CF₄, NF₃, Cl₂, HBr, BCl₃, and/or combinations thereof. Themethods and etchants to remove a single layer stack should not belimited by the embodiments of the present disclosure.

The trimming process includes applying a suitable etching process (e.g.,an isotropic dry etch or a wet etch) on the patterning mask such thatthe patterning mask can be pulled back laterally. The lateral pull-backdimension determines the lateral dimension “a” of each step of thedielectric staircase 560. After patterning mask trimming, one portion ofthe topmost dielectric staircase step 562 is exposed and the otherportion of the topmost dielectric staircase step 562 remains covered bythe patterning mask. The next cycle of etch-trim process resumes withthe etching process.

In some embodiments, the patterning mask trimming process can includedry etching, such as RIE using O₂, Ar, N₂, etc.

In some embodiments, the topmost dielectric staircase step 562 can becovered by the dielectric layer 450. In some embodiments, the topmostdielectric staircase step 562 can further be covered by other dielectricmaterials. A process step of removing the dielectric layer 450 and/orthe other dielectric materials can be added to the etching process ofeach etch-trim cycle to form the dielectric staircase 560.

FIG. 6 illustrates a cross-sectional view of an exemplary structure 600of a three dimensional memory device, according to some embodiments,wherein the structure 600 includes a barrier layer 664 disposed over thestructure 500.

The barrier layer 664 covers the dielectric staircase 560 on both thetop surfaces and sidewalls. In some embodiments, the barrier layer 664can be an optional etch-stop layer. For example, the barrier layer 664can be used as an etch-stop layer for protecting the underlyingstructure during contact hole etching processes. In some embodiments, athickness of the barrier layer 664 on sidewalls can be the same as athickness of the barrier layer 664 on the top surfaces. In someembodiments, the thickness of the barrier layer 664 on sidewalls can bedifferent from the thickness of the barrier layer 664 on the topsurfaces. In some embodiments, the barrier layer 664 can be made ofsimilar material as the dielectric layer 450 using a similar technique.

FIG. 7 illustrates a cross-sectional view of an exemplary structure 700of a three dimensional memory device, according to some embodiments,wherein the structure 700 includes a first insulating layer 768 disposedover the structure 600.

The first insulating layer 768 can be disposed on dielectric staircase560 after forming the barrier layer 664. The first insulating layer 768can be made of any a suitable insulator and can be made of a similarmaterial as the dielectric layer 450 using a similar technique. In someembodiments, the first insulating layer 768 can also includespin-on-glass, a mixture of silicon oxide and dopants (either boron orphosphorous) that is suspended in a solvent solution, and can bedisposed using processes, for example, spin-coating. In someembodiments, the first insulating layer 768 can include a low-kdielectric material, such as carbon-doped oxide (CDO or SiOC or SiOC:H),or fluorine doped oxide (SiOF), etc. The low-k dielectric material canbe disposed by CVD, PVD, sputtering, etc.

In some embodiments, a planarization process, for example RIE etch-backor chemical mechanical polishing (CMP), can be performed to form acoplanar surface, parallel to the surface 330 f of the substrate 330. Insome embodiments, the top surface 768S of the first insulating layer 768can be coplanar with the top surface 664S of the uppermost portion ofthe barrier layer 664. In this example, the barrier layer 664 can beused as a polish-stop.

FIG. 8 illustrates a cross-sectional view of an exemplary structure 800of a three dimensional memory device, according to some embodiments,wherein the structure 800 includes a plurality of memory strings 212through the dielectric film stack 445. The memory strings 212 correspondto the memory strings 212 in FIGS. 2A-2B and FIG. 3. For illustrativepurpose, two memory strings are shown in FIG. 8. Each memory string 212extends through the dielectric film stack 445 of alternating dielectriclayer pairs, and includes a memory film 337 over the inner surface ofmemory strings 212, a channel layer 338 over the memory film 337, and acore filling film 339 surrounded by the channel layer 338. Detailedstructure and method of the NAND memory string is described in theco-pending U.S. patent application, titled “Method for Forming GateStructure of Three-Dimensional Memory Device,” (application Ser. No.16/047,158 and filed on Jul. 27, 2018), which is incorporated herein byreference in its entirety.

FIG. 9 illustrates a cross-sectional view of an exemplary structure 900of a three dimensional memory device (along WL direction), according tosome embodiments, wherein the sacrificial layers 452 are removed and aplurality of horizontal tunnels 970 are formed.

After forming the memory strings 212, a plurality of slit structureopenings can be formed along WL directions (see FIGS. 2A-2B and FIG. 3).These slit structure openings extend through the dielectric film stack445. The sacrificial layers 452 can then be removed from the openings ofthe slit structures 216 along BL direction (perpendicular to WLdirection, e.g., perpendicular to the cross-section shown in FIG. 9).

The sacrificial layers 452 can be removed by any suitable etchingprocess, e.g., an isotropic dry etch or wet etch, that is selective overthe dielectric layers 450, such that the etching process can haveminimal impact on the dielectric layer 450. In some embodiments, thesacrificial layer 452 can be silicon nitride. In this example, thesacrificial layer 452 can be removed by RIE using one or more etchantsof CF₄, CHF₃, C₄F₈, C₄F₆, and CH₂F₂. In some embodiments, thesacrificial layer 452 can be removed using wet etch, such as phosphoricacid.

After removing the sacrificial layers 452, sidewalls of the memory film337 are exposed in the horizontal tunnels 970.

FIG. 10 illustrates a cross-sectional view of an exemplary structure1000 of a three dimensional memory device, according to someembodiments, wherein the structure 1000 includes the film stack 335 ofalternating conductive and dielectric layers (e.g., corresponding to thefilm stack 335 in FIG. 3). The film stack 335 of alternating conductiveand dielectric layers includes conductive layers 1072 sandwiched betweenthe dielectric layers 450. In structure 1000, each staircase step 1076includes two or more conductive and dielectric layer pairs 1074, eachconductive and dielectric layer pair 1074 having one conductive layer1072 and one dielectric layer 450. In FIG. 10, as an example, eachstaircase step 1076 includes two conductive and dielectric layer pairs1074-1 and 1074-2, referred to as an “upper layer pair” and a “lowerlayer pair”, respectively. After disposing the conductive layers 1072inside the plurality of horizontal tunnels, the dielectric staircase 560with alternating dielectric and sacrificial layers is now changed into astaircase structure 1060 with alternating conductive and dielectriclayers.

The conductive layer 1072 can include any suitable conductive materialthat is suitable for a gate electrode, e.g., tungsten (W), aluminum(Al), copper (Cu), cobalt (Co), titanium (Ti), tantalum (Ta), titaniumnitride (TiN), tantalum nitride (TaN), and/or any combination thereof.The conductive material can fill the horizontal tunnel 970 using asuitable deposition method such as CVD, physical vapor deposition (PVD),plasma-enhanced CVD (PECVD), sputtering, thermal evaporation, e-beamevaporation, metal-organic chemical vapor deposition (MOCVD), and/orALD. In some embodiments, the conductive layers 1072 include tungsten(W) deposited by CVD.

In some embodiments, the conductive layer 1072 can also bepoly-crystalline semiconductors, such as poly-crystalline silicon,poly-crystalline germanium, poly-crystalline germanium-silicon and anyother suitable material, and/or combinations thereof. In someembodiments, the poly-crystalline material can be incorporated with anysuitable types of dopant, such as boron, phosphorous, or arsenic. Insome embodiments, the conductive layer 1072 can also be amorphoussemiconductors.

In some embodiments, the conductive layer 1072 can be made from a metalsilicide, including WSi_(x), CoSi_(x), NiSi_(x), or AlSi_(x), etc. Theforming of the metal silicide material can include forming a metal layerand a poly-crystalline semiconductor using similar techniques describedabove. The forming of metal silicide can further include applying athermal annealing process on the deposited metal layer and thepoly-crystalline semiconductor layer, followed by removal of unreactedmetal.

In some embodiments, a gate dielectric layer can be disposed in thehorizontal tunnels 970 prior to the conductive layer 1072 (not shown inFIG. 10) to reduce leakage current between adjacent word lines (gateelectrodes) and/or to reduce leakage current between gate and channel.The gate dielectric layer can include silicon oxide, silicon nitride,silicon oxynitride, and/or any suitable combinations thereof. The gatedielectric layer can also include high-k dielectric materials, such ashafnium oxide, zirconium oxide, aluminum oxide, tantalum oxide,lanthanum oxide, and/or any combination thereof. The gate dielectriclayer can be disposed by one or more suitable deposition processes, suchas CVD, PVD, and/or ALD.

The conductive layers 1072 function as gate electrodes at theintersection with memory strings 212. In FIG. 10, the ten conductivelayers 1072 can form ten gate electrodes for each memory string 212,e.g., TSG 334, LSG 332 and eight control gates 333. Corresponding toeight control gates 333, each memory string 212 can have eight memorycells 340. It is noted that the number of memory strings and memorycells are shown for illustrative purposes in FIG. 10, and can beincreased for higher storage capacity.

After forming gate electrodes, conductive materials can be removed andinsulating materials can be deposited in the openings to form slitstructures 216, separating a memory block into multiple programmable andreadable memory fingers (see FIG. 2A-2B).

In some embodiments, the doped source line regions 344 in portions ofsubstrate 330 can be formed using techniques such as ion implantation(see FIG. 3). In this example, a conductive core can be inserted in theslit structure 216 to form common source contact to the doped sourceline region 344.

Structure 1000 can include other structures, for example, through arraycontact (TAC), TSG cut, common source contact and dummy channelstructure, which are not shown in FIG. 10 for simplicity.

FIG. 11 illustrates a cross-sectional view of an exemplary structure1100 of a three dimensional memory device, according to someembodiments, wherein the structure 1100 includes a plurality of firstcontact holes 1180 in the first insulating layer 768 with a diameter of“d₁”. In FIG. 11, one first contact hole 1180 is shown for eachstaircase step 1076, which is only for illustrative purpose. Multiplefirst contact holes 1180 can be formed on each staircase step 1076. Insome embodiments, there are no first contact holes 1180 on dummystaircase levels.

In some embodiments, photoresist or polymer material can be used as amask layer to etch the first contact holes 1180. Due to the topology ofthe staircase structure, depth “H” of the first contact hole 1180 fromthe top surface to staircase step depends on the location of each step.For a 3D NAND memory with many tiers of word lines, the first contactholes 1180 for the lower staircase steps can be much deeper than thefirst contact holes 1180 for the upper staircase steps. Therefore, thefirst contact hole 1180 for the staircase step 1076 closer to thesurface 330 f of the substrate 330 requires longer etch time than thefirst contact hole 1180 for the staircase step 1076 away from thesurface 330 f of the substrate 330. A selective etching process can beused such that the etching rate of the first insulating layer 768 ishigher than the conductive layer 1072 and/or the barrier layer 664.

In some embodiments, during the etching process for first contact holes1180, the barrier layer 664 can function as an etch-stop layer and canprotect the underlying structure until all the first contact holes 1180are formed on top of the barrier layer 664 for all levels of thestaircase structure 1060. And then the portions of the barrier layer 664inside the first contact holes 1180 can be removed using the same masklayer. In some embodiments, when a gate dielectric layer is disposed onthe conductive layer 1072, the etching also includes removing the gatedielectric layer inside the first contact holes 1180.

The first contact holes 1180 extend through the first insulating layer768, the barrier layer 664, and the optional gate dielectric layer,exposing a portion of the conductive layer 1072 of the upper layer pair1074-1 in each staircase step 1076. In some embodiments, the firstinsulating layer 768 is silicon oxide and the barrier layer is acombination of silicon nitride and silicon oxide. In this example,etching silicon oxide can use anisotropic RIE with chemical etchant, forexample, CF₄, CHF₃, C₂F₆, C₃F₆, and/or any combination thereof. Etchingsilicon nitride can use RIE with chemical etchant, for example, O₂, N₂,CF₄, NF₃, Cl₂, HBr, BCl₃, and/or combinations thereof.

The diameter “d₁” of the first contact holes 1180 is preferably smallerthan the lateral dimension “a” of the staircase structure 1060, and willbe discussed in detail in the subsequent processes.

FIG. 12 illustrates a cross-sectional view of an exemplary structure1200 of a three dimensional memory device, according to someembodiments, wherein the structure 1200 includes a conductive film 1282,disposed over the structure 1100.

In some embodiments, the conductive film 1282 inside the first contacthole 1180 is in direct contact with the conductive layer 1072 of theupper layer pair 1074-1. The conductive film 1282 also covers a sidewallof the first contact hole 1180. The thickness “t₁” of the conductivefilm 1282 at the bottom of the first contact hole 1180 can be the sameor different from the thickness “t₂” on the sidewall. The height of theconductive film 1282 inside the first contact hole 1180 is determined bythe depth “H” of the first contact hole 1180.

The conductive film 1282 can include any suitable conductive material,for example, a metal or metallic compound such as tungsten (W), aluminum(Al), copper (Cu), cobalt (Co), titanium (Ti), tantalum (Ta), titaniumnitride (TiN), tantalum nitride (TaN), and/or any combination thereof.The metal or metallic compound can be disposed using a suitabledeposition method such as CVD, PVD, PECVD, sputtering, thermalevaporation, e-beam evaporation, MOCVD, and/or ALD.

The conductive film 1282 can also be a metal silicide, including WSix,CoSix, NiSix, or AlSix, etc. Metal silicide material can be formed bydisposing a metal layer directly on a polycrystalline silicon layerinside the first contact hole 1180 and then applying a thermal annealingprocess followed by removal of unreacted metal.

In some embodiments, the conductive film 1282 includes a combination ofTiN/W/TiN deposited by CVD.

FIG. 13 illustrates a cross-sectional view of an exemplary structure1300 of a three dimensional memory device, according to someembodiments, wherein the structure 1300 includes a plurality ofconductive rings 1384 and a plurality of ring openings 1386. Theconductive ring 1384 covers a sidewall of the first insulating layer768. Bottom of the conductive ring 1384 contacts the conductive layer1072 of the upper layer pair 1074-1.

The conductive ring 1384 can be formed by removing the conductive film1282 and the conductive layer 1072 from the bottom of the ring openings1386 using anisotropic etching, such as anisotropic RIE. In someembodiments, the conductive film 1282 and the conductive layer 1072 ofthe staircase structure 1060 can be tungsten. In this example, theanisotropic etching to form the conductive ring 1384 can include dryetching, for example ME with a mixture of O₂ and CF₄, CClF₃, or CBrF₃.

Anisotropic RIE can include low-pressure plasma system to increasemean-free path of the ions and reduce random scattering. Duringanisotropic etching, the ions strike the structure 1300 in a verticaldirection, perpendicular to the top surface 330 f of the substrate 330.In some embodiment, the height “H” (shown in FIG. 12) of the conductivefilm 1282 can be greater than the total thickness of “t₁” at the bottomof the first contact holes 1180 and the thickness of the conductivelayer 1072. Therefore, the conductive film 1282 and the conductive layer1072 at the bottom of the first contact holes 1180 can be removed, whilethere is remaining conductive film on the sidewall of the first contacthole 1180, forming the conductive ring 1384.

The thickness “t₃” of the conductive ring 1384 depends on the initialsidewall thickness “t₂” of the conductive film 1282, as well as thesidewall profile of the first contact holes 1180. The thickness “t₃” canfurther depend on the RIE process conditions, for example, total etchtime, ion direction angle, pressure, DC bias voltage and RF power, etc.To reduce parasitic resistance and metal line loading, the conductivering 1384 with a greater thickness “t₃” is preferred. However trade-offbetween memory performance and area is needed for a limited diameter“d₁” of the first contact holes 1180.

For each staircase step 1076, the ring openings 1386 extend through theconductive layer 1072 of the upper layer stack 1074-1, with a smallerdiameter “d₂” than the diameter “d₁” of the first contact holes 1180, asshown in FIG. 13.

FIG. 14 illustrates a cross-sectional view of an exemplary structure1400 of a three dimensional memory device, according to someembodiments, wherein the structure 1400 includes a second insulatinglayer 1488 disposed over the structure 1300.

The second insulating layer 1488 covers the exposed conductive materialsinside the ring openings 1386, e.g., the conductive ring 1384 andsidewalls of the conductive layer 1072 of the upper layer pair 1074-1for each staircase step 1076. The second insulating layer 1488 can bemade of a similar material as the first insulating layer 768 and used asimilar deposition technique.

FIG. 15 illustrates a cross-sectional view of an exemplary structure1500 of a three dimensional memory device, according to someembodiments, wherein the structure 1500 includes a plurality ofinsulating rings 1590 and a plurality of second contact holes 1592. Theinsulating rings 1590 can be formed by etching portions of the secondinsulating layer 1488 and the dielectric layer 450 from the bottom ofthe ring openings 1386 on structure 1400 (in FIG. 14), wherein the layer1488 and 450 can be etched using an anisotropic etching, similar to thetechnique used for forming the conductive ring 1384 except using adifferent etchant for the dielectric material. In some embodiments, thesecond insulating layer 1488 can be silicon oxide. In this example, theetching of silicon oxide can include RIE using fluorine based gases, forexample, carbon-fluorine (CF₄), hexafluoroethane (C₂F₆), CHF₃, or C₃F₆and/or any other suitable gases.

The second contact hole 1592 extends through the second insulating layer1488 and the dielectric layer 450 of the upper layer pair 1074-1 foreach staircase step 1076, exposing the conductive layer 1072 of thelower layer pair 1074-2.

FIG. 16A illustrates a cross-sectional view of an exemplary structure1600 of a three dimensional memory device, according to someembodiments, wherein the structure 1600 includes a plurality ofconductive cores 1694.

The conductive core 1694 can be made of any suitable conductivematerials and can be similar to the conductive film 1282, forming by asimilar technique. The conductive material for the conductive core 1694can be disposed over the structure 1500, filling the second contact hole1592. The conductive core 1694 directly contacts with the conductivelayer 1072 of the lower layer pair 1074-2 of each staircase step 1076.

In some embodiments, a planarization process, such as CMP, can be usedto remove any conductive materials on the top surface 768S of the firstinsulating layer 768.

FIG. 16B illustrates a perspective view of the structures 1600, whereinthe insulating and dielectric layers are omitted for clarity. Theconductive ring 1384 and the conductive core 1694 form a coaxial contactstructure 1696 on the staircase structure 1060. In some embodiments, thestaircase step 1076 of the staircase structure 1060 includes twoconductive and dielectric layer pairs, the upper layer pair 1074-1 andthe lower layer pair 1074-2. The conductive ring 1384 can beelectrically connected to the conductive layer 1072 of the upper layerpair 1074-1 of the staircase step 1076. The conductive core 1694 can beelectrically connected to the conductive layer 1072 of the lower layerpair 1074-2 of the staircase step 1076. The conductive ring 1384 and theinsulating ring 1590 form a conductive and insulating ring pair 1697,corresponding to one of the conductive and dielectric layer pairs of thestaircase step 1076.

FIG. 17A-17D shows another embodiment of the contact structures for thegate electrodes of a three dimensional memory device. In FIG. 17A-17D aportion of the contact structure and staircase structure are illustratedas an example. Similar elements are labeled with the same referencenumbers to compare with the corresponding elements in FIG. 13-16A.

In this example, during the formation of the conductive ring 1384, usingsimilar processes as described in FIG. 13, the etching process can beperformed longer to form a first contact hole 1786, wherein the firstcontact hole 1786 can extend further through the dielectric layer 450 ofthe upper layer pair 1074-1 in the staircase step 1076, exposing theconductive layer 1072 of the lower layer pair 1074-2 (see structure 1710in FIG. 17A).

FIG. 17B shows a cross-sectional view of an exemplary structure 1720 ofa three dimensional memory device, according to some embodiments,wherein the structure 1720 includes the second insulating layer 1488disposed over the structure 1710 on the conductive ring 1384, a sidewallof the dielectric layer 450 and the exposed portion of the conductivelayer 1072 of the lower layer pair 1074-2 of the staircase step 1076.

FIG. 17C shows a cross-sectional view of an exemplary structure 1730 ofa three dimensional memory device, according to some embodiments,wherein the structure 1730 includes an insulating spacer 1790 and acontact hole 1792, wherein the contact hole 1792 extends through thesecond insulating layer 1488 at the bottom, exposing the conductivelayer 1072 of the lower layer pair 1074-2 of the staircase step 1076.

FIG. 17D shows a cross-sectional view of an exemplary structure 1740 ofa three dimensional memory device, according to some embodiments,wherein the structure 1740 includes a conductive core 1794, wherein theconductive core 1974 can be made from a similar material as theconductive core 1694 and formed by a similar technique. In this example,a coaxial contact structure 1796, similar to the coaxial contactstructures 1696 is formed.

Through the coaxial contact structures 1696/1796, the electricalconductive path for the gate electrode of each memory cell can be wiredup to the surface of the wafer, enabling various configurations of wordlines and select gates for the 3D memory in the back-of-line process.

After forming structure 1600/1740, fabrication of 3D memory device canbe resumed with back-end-of-line (BEOL) metal interconnects, and areknown to a person with ordinary skill in the art. In some embodiments, asecond session of word line stack can be added to the structure1600/1740 to further increase the vertical number of memory cells.

In some embodiments, the staircase structure 1060 can include aplurality of staircase steps 1076, each staircase step 1076 having Nnumber of conductive and dielectric layer pairs 1697, wherein N is awhole number no less than two. In this example, there can be N−1 numberof conductive and insulating ring pairs 1697 in addition to a conductivecore 1694. Each conductive and insulating ring pair 1697 includes oneconductive ring 1384 and one insulating ring 1590, wherein theinsulating ring 1590 is disposed to surround a sidewall of theconductive ring 1384 and is configured to electrically isolate theconductive ring 1384 from another conductive ring 1384 or the conductivecore 1694. The conductive core 1694 is located in the center of thecoaxial contact structure 1696. In some embodiments, the conductive core1694 can also include an insulating core that fills possible seams orholes in the conductive core 1694.

The conductive core 1694 and the conductive rings 1384 can be arrangedso that the conductive rings 1384 make electrical contact with theconductive layer 1072 of a corresponding conductive and dielectric layerpair 1697 of the staircase step 1076. An outer conductive ring with alarger diameter can connect to the conductive layer 1072 of an upperconductive and dielectric layer pair of the staircase step 1076. Aninner conductive ring with a smaller diameter can connect to theconductive layer 1072 of a lower conductive and dielectric layer pair ofthe staircase step 1076. The upper conductive and dielectric layer pairis farther away from the substrate, whereas the lower conductive anddielectric layer pair is closer to the substrate. The conductive core1694 can connect to the bottommost conductive layer, e.g., the pairclosest to the substrate, within the staircase step 1076 of N number ofconductive and dielectric layer pairs.

In some embodiments, the conductive rings 1384 extend through the firstinsulating layer 768 to contact the conductive layer 1072 of thestaircase structure 1060. In some embodiments, the conductive rings 1384also extend through the barrier layer 664 to contact the conductivelayer 1072 of the staircase structure 1060. In some embodiments, a gatedielectric layer can be disposed on the conductive layer 1072. In thisexample, the conductive rings 1384 extend further through the gatedielectric layer to contact the conductive layer of the staircasestructure 1060.

In some embodiments, the insulating ring 1590 of the conductive andinsulating ring pair 1697 can be disposed to surround a sidewall of theconductive layer of the staircase structure in addition to the sidewallof the conductive ring. In some embodiments, the insulating ring can bedisposed on a sidewall of the dielectric layer of the staircase step ofN number of conductive and dielectric layer pairs (similar to thestructure shown in FIG. 17D).

By using the coaxial contact structure to connect two or more conductivelayers of the staircase structure, the number of staircase steps can bereduced and thereby the overall lateral dimension of the staircasestructure can be reduced. Accordingly the area of staircase region 210(shown in FIG. 2A) can be greatly reduced, and higher density memorystorage can be achieved.

FIG. 18 illustrates an exemplary method 1800 for forming staircase andcontact structures for a three-dimensional memory array, according tosome embodiments. The process steps of the method 1800 can be used toform memory device structures illustrated in FIGS. 4-16A. The processsteps shown in method 1800 are not exhaustive and other process stepscan be performed as well before, after, or between any of theillustrated process steps. In some embodiments, some process steps ofexemplary method 1800 can be omitted or include other process steps thatare not described here for simplicity. In some embodiments, processsteps of method 1800 can be performed in a different order and/or vary.

At process step 1810, a dielectric film stack is disposed on asubstrate. The dielectric film stack can be the dielectric film stack445 in FIG. 4, with alternating dielectric (first dielectric) andsacrificial (second dielectric) layers. The dielectric and sacrificiallayers are similar to the dielectric layer 450 and the sacrificial layer452 in FIG. 4 and can be disposed using a similar technique. Thedielectric layer and the sacrificial layer below are called analternating dielectric layer pair.

At process step 1815, a dielectric staircase is formed in the dielectricfilm stack. An example of the dielectric staircase is shown as thedielectric staircase 560 in FIG. 5, wherein the dielectric staircaseincludes a plurality of staircase layers, e.g., staircase steps. Eachstaircase step includes two or more alternating dielectric layer pairs.As an example, FIG. 5 depicts a dielectric staircase with twoalternating dielectric layer pairs. The plural steps of the dielectricstaircase can be formed by applying a repetitive etch-trim process onthe dielectric film stack. First, a patterning mask is disposed andpatterned on the dielectric film stack. Then, portions of the dielectricfilm stack can be exposed and etched in a direction perpendicular to amain surface of the substrate until portions of the two dielectric layerpairs are removed. Afterwards, the patterning mask is trimmed laterally,in a direction parallel to the main surface of the substrate. Theetching and trimming processes can be repeated until a dielectricstaircase step closest to the main surface of the substrate is formed.Finally, the patterning mask can be removed.

At process step 1820, a barrier layer is disposed over the dielectricstaircase, wherein the barrier layer can be the barrier layer 664 inFIG. 6 and can be made of a similar material and formed using a similartechnique.

At process step 1825, a first insulating layer is disposed over thedielectric staircase.

In some embodiments, the first insulating layer is disposed on thebarrier layer. The first insulating layer can be the first insulatinglayer 768 in FIG. 7. Next, a planarization process, such as chemicalmechanical polishing (CMP) or reaction-ion-etching, can be performed toform a coplanar surface. An example of the structure is shown in FIG. 7.

At process step 1830, a plurality of memory strings are formed in thedielectric film stack with alternating dielectric layer pairs. Thememory string is similar to the memory string 212 in FIG. 8, andincludes a memory film, a channel layer and a core filling film.

At process step 1835, the dielectric film stack of alternatingdielectric layer pairs is patterned to form a plurality of slitopenings. The slit openings extend horizontally along the dielectricstaircase and extend vertically through the dielectric film stack. Next,the sacrificial layers of the dielectric staircase are removedhorizontally from the slit openings to form a plurality of horizontaltunnels in the staircase structure. After removing the sacrificiallayers, the memory film of the memory strings are exposed inside theplurality of tunnels. FIG. 9 shows an example of the structure afterremoving the sacrificial layers.

At process step 1840, conductive materials are disposed inside thehorizontal tunnels, forming a staircase structure with alternatingconductive and dielectric layers, which is similar to the staircasestructure 1060 in FIG. 10. The conductive layer can be made of a similarmaterial as the conductive layer 1072 and can be disposed using asimilar technique. In some embodiments, a gate dielectric layer can bedisposed on sidewalls of the horizontal tunnels prior to the conductivelayer deposition, wherein the gate dielectric layer includes high-kdielectric material, silicon oxide, silicon nitride or siliconoxynitride. After replacing the sacrificial layer with the conductivelayer, the staircase structure include a plurality of staircase steps,each having two conductive and dielectric layer pairs, e.g., upper layerpair and lower layer pair.

At process step 1845, the first insulating layer is patterned, forming aplurality of first contact holes. The first contact holes extend throughthe first insulating layer and the optional barrier layer and expose theconductive layer of upper layer pair of the staircase step. The firstcontact holes 1180 in FIG. 11 is an example of the plurality of firstcontact holes.

At process step 1850, a conductive film is disposed over the staircasestructure. The conductive film is disposed on the exposed portion of theconductive layer of the staircase structure and also on a sidewall ofthe first contact hole. The conductive film can be the conductive film1282 in FIG. 12, and can be disposed using a similar technique.

At process step 1855, the conductive film at the bottom of the firstcontact holes is removed through anisotropic etching, forming conductiverings along a sidewall of the first insulating layer. Examples of theconductive rings are shown in FIG. 13 as the conductive rings 1384. Theconductive rings can be made of a similar material as the conductiverings 1384 and be formed using a similar technique.

At process step 1860, a second insulating layer is disposed over thestaircase structure on the conductive rings. The second insulating layeris similar to the second insulating layer 1488 in FIG. 14 and can bemade of a similar material using a similar technique.

At process step 1865, a plurality of insulating rings are formed byanisotropic etching of the second insulating layer. The insulating ringsurrounds a sidewall of the conductive ring as well as an exposedportion of the conductive layer of the upper layer pair in the staircasestep. During the etching process, the dielectric layer of the upperlayer pair can also be removed and the conductive layer of the lowerlayer pair can be exposed. A plurality of second contact holes areformed accordingly. An example of the insulating rings and secondcontact holes is shown in FIG. 15, as the insulating rings 1590 and thesecond contact hole 1592.

At process step 1870, a plurality of conductive cores are formed insidethe second contact holes. The conductive core contact the conductivelayer of the lower layer pair in the staircase step. The conductive corecan be the conductive core 1694 in FIG. 16, and can be formed using asimilar technique. A planarization process, for example CMP, can be usedto form a coplanar surface. From top down, the conductive ring,insulating ring and the conductive core form a coaxial contactstructure. The coaxial contact structures can provide electricalconnections to each of the conductive layer of the staircase structure.With two conductive and dielectric layer pairs per staircase step, thenumber of contact structures can be reduced to half, saving the area ofstaircase region. From these coaxial contact structures,back-end-of-line processes can be resumed with metal interconnect linesto form a functional 3D NAND memory.

In some embodiments, the staircase structure 1060 can include aplurality of staircase steps 1076, each staircase step 1076 having Nnumber of conductive and dielectric layer pairs 1697, wherein N is awhole number no less than two. In this example, there can be N−1 numberof conductive and insulating ring pairs 1697 in addition to a conductivecore 1694. N−1 number of conductive and insulating ring pairs and theconductive core can be formed using similar processes as described inprocess steps 1845-1870.

In some embodiments, to form the N−1 number of conductive and insulatingring pairs, a first contact hole can be formed to expose a conductivelayer in one of the N number of conductive and dielectric layer pairs ofthe staircase step in the staircase structure. Then a conductive filmcan be disposed on a sidewall of the contact hole and the exposedconductive layer. Next, the conducive film and a portion of theconductive layer from the bottom of the first contact hole can beremoved to form a conductive ring, wherein a bottom of the conductivering is formed to contact the conductive layer in one of the N number ofconductive and dielectric layer pairs of the staircase step in thestaircase structure. In some embodiments, the dielectric layer of thenext conductive and dielectric layer pair can also be removed during theetching process for the conductive ring.

In some embodiments, a second insulating layer can be disposed in thefirst contact hole and the second insulating layer can then be removedfrom the bottom of the first contact hole to form an insulating ring,thereby surrounding a sidewall of the conductive ring and a sidewall ofthe conductive layer is exposed in the first contact hole. The cyclicprocesses for the next conductive and insulating ring pairs resume,starting by forming a second contact hole to expose the next conductivelayer in the staircase step, etc.

In some embodiments, the conductive core of the coaxial contactstructure can be formed last. After forming a contact hole to expose thebottommost conductive layer, e.g., closest to the substrate, in thestaircase step of N number of conductive and dielectric layer pairs, aconductive material can be disposed to fill the contact hole, and aconductive core can be formed to contact the bottommost conductive layerusing a planarization process, such as chemical mechanical polishing.The conductive material outside the contact hole can be removed and thestructure can be formed with a coplanar surface.

Accordingly, a staircase structure is formed with a plurality ofstaircase steps, each step having N number of conductive layers. Aplurality of coaxial contact structures are formed on the staircasestructure. Each coaxial contact structure can provide N number ofconductive paths to connect to the N number of the conductive layers,and to the word lines for the vertically stacked memory strings. Bysharing contact structures, the lateral dimension of the staircasestructure can be greatly reduced, the storage density of the 3D memorydevices can be increased.

Various embodiments in accordance with the present disclosure provide a3D memory device with smaller die size, higher device density, andimproved performance compared with other 3D memory devices.

Accordingly, various embodiments of three-dimensional memory device andmethods of making the same are described in the present disclosure.

In some embodiments, the three-dimensional memory structure includes afilm stack disposed on a substrate, wherein the film stack includes aplurality of conductive and dielectric layer pairs, each conductive anddielectric layer pair having a conductive layer and a first dielectriclayer. The three-dimensional memory structure also includes a staircasestructure formed in the film stack, wherein the staircase structureincludes a plurality of steps, each staircase step having two or moreconductive and dielectric layer pairs. The three-dimensional memorystructure further includes a plurality of coaxial contact structuresformed in a first insulating layer over the staircase structure, whereineach coaxial contact structure includes one or more conductive andinsulating ring pairs and a conductive core, wherein each conductive andinsulating ring pair includes a conductive ring and an insulating ring.

In some embodiments, a method for forming a three-dimensional memorystructure includes disposing a dielectric film stack on a substrate,wherein the dielectric film stack includes a plurality of alternatingdielectric layer pairs, each alternating dielectric layer pair having afirst dielectric layer and a second dielectric layer different from thefirst dielectric layer. The method also includes forming a dielectricstaircase in the dielectric film stack, wherein the dielectric staircaseincludes a plurality of steps, each dielectric staircase step having twoor more alternating dielectric layer pairs. The method further includesdisposing a first insulating layer on the dielectric staircase, forminga plurality of memory strings in the dielectric film stack, andreplacing the second dielectric layers with conductive layers to form astaircase structure with a plurality of steps, wherein each staircasestep includes two or more conductive and dielectric layer pairs, eachconductive and dielectric layer pair having a conductive layer and thefirst dielectric layer. The method also includes forming a plurality ofcoaxial contact structures on the staircase structure.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present disclosure that others can, byapplying knowledge within the skill of the art, readily modify and/oradapt, for various applications, such specific embodiments, withoutundue experimentation, and without departing from the general concept ofthe present disclosure. Therefore, such adaptations and modificationsare intended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the disclosure and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the disclosure andguidance.

Embodiments of the present disclosure have been described above with theaid of functional building blocks illustrating the implementation ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed.

The Summary and Abstract sections can set forth one or more but not allexemplary embodiments of the present disclosure as contemplated by theinventor(s), and thus, are not intended to limit the present disclosureand the appended claims in any way.

The breadth and scope of the present disclosure should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. A three-dimensional memory structure, comprising:a film stack disposed on a substrate, wherein the film stack comprises aplurality of conductive and dielectric layer pairs, each conductive anddielectric layer pair comprising a conductive layer and a firstdielectric layer; a staircase structure formed in the film stack,wherein the staircase structure comprises a plurality of steps, eachstaircase step comprising two or more conductive and dielectric layerpairs; and a plurality of coaxial contact structures formed in a firstinsulating layer over the staircase structure, wherein each coaxialcontact structure comprises one or more conductive and insulating ringpairs and a conductive core, wherein each conductive and insulating ringpair comprises a conductive ring and an insulating ring.
 2. Thethree-dimensional memory structure of claim 1, wherein each conductivering contacts the conductive layer of a corresponding conductive anddielectric layer pair of the staircase step.
 3. The three-dimensionalmemory structure of claim 2, wherein each coaxial contact structurecomprises at least an outer conductive ring and an inner conductivering; and the outer conductive ring corresponds with an upper conductiveand dielectric layer pair of the staircase step, wherein the outerconductive ring comprises larger diameter, and the upper conductive anddielectric layer pair is farther away from the substrate.
 4. Thethree-dimensional memory structure of claim 2, wherein each coaxialcontact structure comprises at least an outer conductive ring and aninner conductive ring; and the inner conductive ring corresponds with alower conductive and dielectric layer pair of the staircase step,wherein the inner conductive ring comprises smaller diameter, and thelower conductive and dielectric layer pair is closer to the substrate.5. The three-dimensional memory structure of claim 1, wherein theconductive core contacts the conductive layer closest to the substratein the staircase step of two or more conductive and dielectric layerpairs.
 6. The three-dimensional memory structure of claim 1, wherein theinsulating ring of the conductive and insulating ring pair is disposedto surround a sidewall of the conductive ring and a sidewall of theconductive layer of the staircase structure, wherein the insulating ringis configured to electrically isolate the conductive ring from anotherconductive ring or the conductive core.
 7. The three-dimensional memorystructure of claim 6, wherein the insulating ring is disposed on asidewall of the first dielectric layer of the staircase step of two ormore conductive and dielectric layer pairs.
 8. The three-dimensionalmemory structure of claim 1, further comprising a barrier layer,disposed between the first insulating layer and the staircase structure,and the plurality of coaxial contact structures extending through thebarrier layer.
 9. The three-dimensional memory structure of claim 1,further comprising: a gate dielectric layer on the conductive layer, andthe conductive rings extending through the gate dielectric layer tocontact the conductive layers of the staircase structure.
 10. A methodfor forming a three-dimensional memory structure, comprising: disposinga dielectric film stack on a substrate, wherein the dielectric filmstack comprises a plurality of alternating dielectric layer pairs, eachalternating dielectric layer pair comprising a first dielectric layerand a second dielectric layer different from the first dielectric layer;forming a dielectric staircase in the dielectric film stack, wherein thedielectric staircase comprises a plurality of steps, each dielectricstaircase step comprising two or more alternating dielectric layerpairs; disposing a first insulating layer on the dielectric staircase;forming a plurality of memory strings in the dielectric film stack;replacing the second dielectric layers with conductive layers to form astaircase structure with a plurality of steps, wherein each staircasestep comprises two or more conductive and dielectric layer pairs, eachconductive and dielectric layer pair comprising a conductive layer andthe first dielectric layer; and forming a plurality of coaxial contactstructures on the staircase structure.
 11. The method of claim 10,wherein forming the coaxial contact structure comprises: forming aconductive and insulating ring pair for each conductive and dielectriclayer pair of the staircase step in the staircase structure, wherein theconductive and insulating ring pair comprises a conductive ring and aninsulating ring.
 12. The method of claim 11, wherein forming theconductive ring comprises: forming a first contact hole to expose theconductive layer in one of the two or more conductive and dielectriclayer pairs of the staircase step in the staircase structure; disposinga conductive film on a sidewall of the contact hole and the exposedconductive layer; and removing the conducive film and a portion of theconductive layer from a bottom of the first contact hole to form aconductive ring, wherein a bottom of the conductive ring is formed tocontact the conductive layer in one of the two or more conductive anddielectric layer pairs of the staircase step in the staircase structure.13. The method of claim 12, wherein forming the conductive ring furthercomprises etching the first dielectric layer of the next conductive anddielectric layer pair.
 14. The method of claim 11, wherein forming theinsulating ring comprises: disposing a second insulating layer in afirst contact hole; removing the second insulating layer from the bottomof the first contact hole; and forming the insulating ring surrounding asidewall of the conductive ring and a sidewall of the conductive layerof one of the two or more conductive and dielectric layer pairs in thestaircase step in the staircase structure; and forming a second contacthole to expose the next conductive layer in the staircase step.
 15. Themethod of claim 10, wherein forming the coaxial contact structurefurther comprises: forming a contact hole to expose the conductive layerclosest to the substrate in the staircase step of two or more conductiveand dielectric layer pairs; disposing a conductive material to fill thecontact hole; and forming a conductive core to contact the conductivelayer closest to the substrate in staircase step of two or moreconductive and dielectric layer pairs.
 16. The method of claim 15,further comprising performing a planarization process to remove theconductive material outside the contact hole and form a coplanarsurface.
 17. The method of claim 10, further comprising disposing abarrier layer on the dielectric staircase prior to the first insulatinglayer.
 18. The method of claim 10, wherein forming the plurality ofdielectric staircase steps, each having two or more dielectric layerpairs, comprises: disposing a patterning mask on the dielectric filmstack; etching exposed portions of the dielectric film stack in adirection perpendicular to a main surface of the substrate untilportions of the two or more dielectric layer pairs are removed; trimmingthe patterning mask laterally, in a direction parallel to the mainsurface of the substrate; repeating the etching and the trimming until adielectric staircase step closest to the main surface of the substrateis formed; and removing the patterning mask.
 19. The method of claim 10,wherein replacing the second dielectric layers with the conductivelayers to form the staircase structure, comprises: forming one or moreslit structure openings, extending horizontally along the dielectricstaircase, wherein the slit structure openings penetrate verticallythrough the dielectric film stack; removing the second dielectric layersof the dielectric staircase to form a plurality of horizontal tunnels;and disposing the conductive layers inside the plurality of horizontaltunnels.
 20. The method of claim 19, further comprising: disposing agate dielectric layer on sidewalls of the horizontal tunnels prior todisposing the conductive layer, wherein the gate dielectric layercomprises high-k dielectric material, silicon oxide, silicon nitride orsilicon oxynitride.